Some neurons in the vestibular nuclei project monosynaptically within the brain stem to motoneurons of the extraocular muscles, and some project caudally to the spinal cord. The purpose of this project is to determine where within the vestibular nuclei are located the neurons that project to the motoneuron pool of each of the twelve extraocular muscles, and where are the spinally projecting neurons. The method of approach is electrophysiological: action potentials of neurons in the vestibular nuclei will be recorded extracellularly using glass micropipette electrodes, and electrical stimuli will be applied at various portions of the extraocular motoneuron complex and in the spinal cord to attempt to identify the target to which the neuron projects by the presence of an antidromic response to electrical stimulation applied at a particular location. Subsequently, histological techniques will be used both to verify the location of the stimulating electrodes and to identify each recording site within the vestibular nuclei. In the first stage of the experiment the area containing neurons that project to the ipsilateral side will be compared with that which contains contralaterally projecting neurons. In the second stage it will be determined within each area of unilateral projection where are the neurons located that go to each of the motoneuron pools. To accomplish these two goals it is necessary to insert only one antidromic stimulating electrode in each animal. In the final stage of the experiment two or more antidromic stimulating electrodes will be implanted in each animal to determine with finer precision the differential localization of neurons having different efferent projection, and also to investigate the possibility that a single neuron branches so as to project to two different targets. The information to be acquired during the course of this experiment should increase our understanding of the micro-organization of the neuronal mechanisms underlying vestibular reflexes.